neural spine bifurcation in sauropod dinosaurs of the - PalArch

Wedel & Taylor, Neural Spine Bifurcation in Sauropods PalArch’s Journal of Vertebrate Palaeontology, 10(1) (2013)
Introduction
Among tetrapods, sauropod dinosaurs are unusual
in that many taxa have deeply bifid neural
spines in their presacral vertebrae. Many mammals
have shallowly bifid spines in their cervical
vertebrae, but usually only the neurapophysis
is divided, whereas in sauropods the division is
more extensive. In the most extreme cases the
midline cleft extends to the roof of the neural
canal, completely dividing the neural spine into
bilaterally paired metapophyses (figure 1). Bifid
presacral neural spines evolved several times
independently in sauropods, and are present
in some mamenchisaurids, all known diplodocids
and dicraeosaurids, the basal macronarian
Camarasaurus, the basal somphospondyls
Euhelopus, Erketu, and Qiaowanlong, and the
derived titanosaur Opisthocoelicaudia (Wilson
& Sereno, 1998; Ksepka & Norell, 2006; You &
Li, 2009; figure 2). In addition, the tips of the
proximal caudal neural spines are often weakly
bifid in diplodocids (e.g. Diplodocus carnegii
CM 84/94, Hatcher, 1901: plate 9). In contrast,
non-pathological bifid neural spines are uncommon
in extant tetrapods, and are limited to the
cervical vertebrae in certain large-bodied, longnecked
birds (Rhea, Tsuihiji, 2004: figure 2b; Casuarius,
Schwarz et al., 2007: figure 5b; Dromaius,
Osborn 1898: figure 1; Theristicus, Tambussi
et al., 2012: 7; also in the recently extinct Dromornithidae,
Gastornithidae, and Phorusracidae,
Tambussi et al. 2012: 7), the thoracic vertebrae
in some bovids (e.g. zebu Bos indicus, Mason &
Maule, 1960: 20), and the lumbar vertebrae of sirenians
(Kaiser, 1974). Cervical neural spines in
humans and many other mammals have paired
tubercles at their tips (Kapandji, 2008: 190-
191; Cartmill et al., 1987: figure 2-3a; figure 3).
They are therefore sometimes described as
being bifid (e.g. White & Folkens, 2000: 145).
The appearance of bifurcation is caused by the
outgrowth of bone at the spine tip to anchor
the large transversospinalis muscles. This is
a different phenomenon from the non-union
of the endochondral portions of the vertebral
spine, which occurs pathologically in humans
(and presumably all other vertebrates) as spina
bifida cystica and spina bifida occulta (Barnes,
1994: 46-50 and figures 3.5 and 3.6).
The developmental underpinnings of bifid
neural spines in sauropods are not well under-
Figure 1. A cervical vertebra of Apatosaurus ajax YPM 1860 showing complete bifurcation of the neural spine into paired
metapophyses. In dorsal (top), anterior (left), left lateral (middle), and posterior (right) views.
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